1. Field of the Invention
The present invention relates to a stamper plate attaching/detaching device of an injection mold for forming an optical disc substrate for a CD-ROM or the like.
2. Description of the Related Art
FIG. 4 is a cross-sectional view of an essential section of an optical disc substrate injection mold which employs a conventional cavity ring. A stamper plate attaching/detaching device for the optical disc substrate injection mold holds the stamper plate by using the cavity ring and a means for holding the central hole of the stamper plate. A hole 5 for receiving a means 2 for holding the inner circumferential rim of the stamper plate is provided at the center of a cavity side disc cavity plate 4. The cavity side disc cavity plate 4 is fixed to a cavity side base plate 13. A sprue bush 8, which has a resin passage 9 at the center thereof, is provided at the center of the aforesaid members.
To attach a stamper plate 1, the cylindrical body of the stamper plate holding means 2 is inserted in the central hole in the disc-shaped stamper plate 1 to receive the stamper plate 1. A flange 3 of the stamper plate holding means 2 holds the inner circumference rim of the stamper plate 1 against the cavity plate 4. The stamper plate holding means 2 is inserted in the central hole 5 of the cavity side disc cavity plate 4 and is fixed by a known method. Then, a cavity ring 6 is attached to the cavity side disc cavity plate 4 by mounting bolts 7. This procedure is reversed when detaching the stamper plate 1.
A movable disc cavity plate 10 is fixed to a movable base plate 14. A punch 12 for punching a central hole in a disc substrate is inserted in a central hole 11 of the movable disc cavity plate 10.
The conventional device described above is designed to mechanically attach or detach the outer circumferential rim of the stamper plate 1 by installing or removing the cavity ring 6 by installing or removing the mounting bolts 7; it is extensively used because it enables the stamper plate 1 to be securely attached or detached, thus leading to high reliability. The stamper plate 1, however, must be attached or detached with the mold installed in a limited space in a molding machine. This makes it extremely difficult and also dangerous to detach or attach the stamper plate. Hence, there has been a high demand for an improved method of attaching and detaching the stamper plate.
To solve the problem stated above, there has been proposed an injection mold for making optical disc substrates which is equipped with a vacuum suction type stamper plate holder (refer, for example, to Japanese Patent Publication No. 2-60502). In this type of metal mold, the stamper plate can be easily removed from the disc cavity plate by stopping vacuum suction. This metal mold, however, has the following shortcoming: the cavity of the metal mold for injection-molding optical disc substrates is frequently put in a vacuum state by vacuum suction before filling the cavity with melted resin in order to improve the uniformity of the density of the melted resin charged in the cavity. In this case, the rear surface of the stamper plate and the cavity are vacuumized to the same level and the suction force on the rear surface of the stamper plate is not effective. This leads to a danger in that the stamper plate comes off a cylindrical bush due to the vibration of the molding machine operating at high speed or if a shock is applied to the molding machine. Another problem is the need for driving a vacuum pump at all times as long as the stamper plate is mounted, even when the molding operation is interrupted.
There is also a magnetic attraction type metal mold for molding disc substrates as disclosed in Japanese Patent Laid-Open No. 5-185475. This type of metal mold makes use of the fact that the stamper plate is made of nickel featuring permeability; a solenoid coil electromagnet or magnet shaped like a disc is buried in the entire surface of the stamper plate mounting surface of the disc cavity plate on the stamper plate side so as to magnetically attract the stamper plate. The design is advantageous in that the structure of the metal mold is simpler since it does not use a vacuum pump.
This type of metal mold, however, has the following shortcomings. Firstly, the disc cavity plate uses martensite-based stainless steel which is subjected to heat treatment to hardness of H.sub.R C 55 to 60 and also to corrosion-resistance treatment to hardness of H.sub.V 1800 or more in order to protect the surface of the disc cavity plate from scratches and corrosion. In addition, the roughness of the surface must be finished to 10 nm or less. Therefore, it is technically extremely difficult to satisfy the above requirements in embedding a soft magnet material in the surface of the disc cavity plate.
Secondly, the heat controlling techniques including that for maintaining a uniform cooling rate for the disc cavity plate, are as important as the aforesaid selection of the material in ensuring the quality of the disc substrates. It is extremely difficult to assure satisfactory molding functions in the presence of the magnet between the cooling water channel provided at the back surface of the disc cavity plate and the surface of the disc cavity plate.